Description

Because of the way ModelRename also triggers AlterField on the foreign keys of other models, it doesn't work in reverse, as that way round the foreign keys are repointed to the old model before it is re-created.

Since some backends (e.g. PostgreSQL) repoint constraints along with renames, we need to make sure that no AlterField is triggered with a rename and then have RenameModel deal with the FKs on MySQL (and perhaps Oracle?)

I'm marking model renames as irreversible for now in order to move this away from release blocker status; there's other irreversible core operations, and we can address this if we get time before the final.